Communication systems are used to deliver information. Communication systems may be used to send information (e.g., data) from one electronic system to another electronic system. The sending electronic system typically includes a transmitter for processing data for transmission across a communication channel. The receiving electronic system includes a receiver for processing signals received across the communication channel. Typically, data is generated by the sending electronic system and forwarded to the transmitter, and the transmitter processes the data for transmission across the channel. Similarly, the receiver processes the received signal and forwards the data to the receiving electronic system.
Transmitters often include baseband circuits that receive data and perform data processing on the data to prepare the data for transmission. The processing may include encoding the data into one or more digital signals, such as a quadrature amplitude modulation (“QAM”) signal, for example. Baseband processing is typically limited to processing digital signals that are centered on zero frequency. In some applications, the digital signals are then up-converted to higher frequencies by an up-converter to create a composite signal. Multiple digital signals may be up-converted to multiple different frequencies. The combined signals form the composite signal to be transmitted across the communication channel, and the up-converted digital signals correspond to channels of a composite signal.
After up-conversion, the digital signals may be converted to analog signals using a digital-to-analog converter. Finally, an analog frontend typically drives the analog signals onto the communication channel. The analog front end may include analog amplifiers and other circuits tailored to condition the analog signal for transmission onto a particular communication channel. Example communication channels include cables (e.g., coaxial cables), optical fibers, twisted pairs, and the air (e.g., for RF transmission).
One problem with some transmitters occurs when the composite signal is converted from digital to analog by a digital to analog converter (“DAC”). Circuitry in the DAC may distort the composite signal as the digital signal is converted to an analog signal. Distortion may result in unwanted spectral artifacts at particular frequencies. For example, in a cable transmitter application, distortion from a DAC may cause an unwanted frequency component at (fs/2)−2fo, where fs is the sample frequency and fo is the center frequency. Such distortion is generally undesirable, and in some cases such distortion can cause the system to fail compliance testing.
More generally, as the number of channels increases in communication systems, additional circuits are required to process the digital signals. Such additional circuits result in an undesirable increase in power consumption. Furthermore, additional circuits occupy more space on an integrated circuit and increase the cost of the system.